The mound is round and about eight metres across. Originally the heating source was to be hot air supplied by home made coroplast collectors so there were large land drainage pipes for heating. There are two 1.5" pipes for heat extraction. I had to abandon the hot air idea because there was too much heat loss in transmission of heat even before reaching the store. Now the inner hotter water pipe is used to heat and the mainly the cooler outer one for heat extraction. This year the two had to be used at the end of the winter because the temperature dropped to about 8C and the combined flow reduced the temperature drop. I hope to be able to keep the temperature above 15C in future. The heating rate is fairly predictable and so is heat extraction but the heat loss from the store is hard to predict. Over the first winter there was minimal heat loss because the store temperature wasn't much higher than ambient temperature.
In Ireland other system are being built with the seasonal heat store under the house but I think that there could be problems of excessive summer temperatures in a hotter climate. The advantage of the heat pump rather than direct heat transfer is the temperature range can be much wider and using lower temperatures reduces storage losses.
One modification I am making shortly is to put a VAWT on top of the mound. This is mostly for decoration but in windy weather it will put a little heat into the store.
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